Host–Guest Exchange of Viologen Guests in Porphyrin Cage Compounds as Studied by Selective Exchange Spectroscopy (1D EXSY) NMR

Abstract

Dynamics in complexes of porphyrin cage compounds and viologen-derived guest molecules are investigated by selective exchange NMR spectroscopy (1D EXSY). Exchange rates were found to be independent of excess guest concentration, revealing a dissociative exchange mechanism, which is accompanied by negative activation entropies, indicating significant reorganization of the host-guest complex during dissociation. Nonsymmetric viologen guests with bulky head groups had more unidirectional binding and slower exchange rates than guests with less-bulky head groups. Thermodynamic and kinetic studies revealed that the exchange process is primarily driven by the thermodynamics of binding and that guest binding can be influenced by introducing steric and electronic groups on the host . Exchange studies with guests bearing a polymer chain revealed that both slippage and full dissociation takes place and the rate constants for both processes were determined. The slippage rate constant revealed that for smaller guests exchange takes place nearly exclusively under thermodynamic control.

Identification of Photocatalytic Alkaloids from Coptidis Rhizome by an Offline HPLC/CC/SCD Approach

“…Several approaches are being pursued to address this limitation: (i) Host–guest chemistry. Cucurbituril [ 27 ], porphyrins [ 28 ], crown ether [ 29 ], calixarenes [ 30 ], cyclodextrins [ 31 ], co-crystals [ 32 , 33 ], MOF [ 34 ], and the recently reported mechanically interlocked structures [ 35 ] have proven abilities to capture a broad range of different organic molecules. (ii) Hydrogen-bonded frameworks.…”

113Cd as a Probe in NMR Studies of Allosteric Host‐Guest‐Ligand Complexes of Porphyrin Cage Compounds

“…[24] We investigated the kinetic factors of the allosteric effect of the binding in Cd1 with the help of 1D Exchange Spectroscopy (EXSY) NMR experiments. [25] These experiments require the exchange process to be slow on the chemical shift timescale, i. e., individual signals must be present for both the free and bound states, not coalesced. This condition was satisfied for the complex of Cd1 with Me 2 V but not for the complex of Cd1 with DMI.…”

“…The negative activation entropies are likely associated with the solvation of Me 2 V once it exits the cavity. [25] Prior to Me 2 V dissociation, Cd1 in the absence of tbupy likely has an acetonitrile bound. We propose that after guest dissociation, this outside acetonitrile molecule is replaced by an acetonitrile molecule that coordinates on the inside of the cage, thereby solvating it to some extent.…”

Proton Exchange of Intracyclic NH Groups of Deuteroporphyrin IX Dimethyl Ester and Water Molecules according to the Data of Exchange Spectroscopy (EXSY)

“…Измерение констант скорости в методах EXSY по временной зависимости кросспиков, кроме того, является общепринятым методом определения констант химического обмена. [11,13,16,22,23] Ранее обменные процессы в этой же системе нами были исследованы при помощи DOSY [8] и определены константы скорости процессов переноса протона с молекулы воды на молекулу порфирина и переноса протона с внутрициклической группы NH порфирина на молекулу воды, а также соответствующие энергии активации (Схема 1). Интересно, что k NH и k W отличаются между собой и не совпадают с определенной в настоящей работе k ЕХ .…”

“…В то же время метод DOSY позволяет исследовать только те процессы, которые связаны с изменением коэффициента самодиффузии D протонов. Полную информацию об обменных процессах можно получить при помощи метода обменной спектроскопии EХSY, который широко используется для изучения обменных процессов, [11][12][13][14][15][16][17][18][19][20] в том числе протонного обмена. [11,12,15] Методика двухмерной спектроскопии EXSY основана на том, что с помощью двух ( π /2)-импульсов, разделенных временем эволюции t 1 , создаются неравновесные состояния намагниченности одновременно для всех частот спектра ЯМР исследуемого соединения, затем через время смешивания t m подается анализирующий импульс.…”